FIELD OF THE INVENTION
The present invention relates to an electric power steering apparatus.
An electric power steering apparatus for a vehicle, for instance, is disclosed in Japanese Patent Unexamined Publication No. Hei. 8-207798. In the apparatus, while the wheels of the vehicle are steered, a steering torque is detected so as to assist the manual power imparted to a steering wheel. In such a structure, an input shaft receives a steering force from the steering wheel and an output shaft transmits the steering force to the wheels. The input shaft and the output shaft are connected by a torsion bar, and the steering torque necessary for control can be detected by detecting the torsion of such a torsion bar.
In a case where the wheel has come into contact with the shoulder of a road, or the steering wheel has been rotated to a rack end during sudden steering, a large force is transmitted between the input shaft and the output shaft, so that there is a possibility that the large force is applied to the torsion bar. In such a case, if the torsion bar undergoes plastic deformation beyond its elastic limit, the zero reference point of the steering torque changes, so that the control function of the electric power steering apparatus is hampered. Accordingly, a limiting member is generally provided for limiting the relative rotation of the input shaft and the output shaft, thereby functioning to prevent plastic deformation from occurring in the torsion bar even if a large force is applied to it.
Such a limiting member comprises a projecting portion and a recessed portion, the projecting portion being formed on the output shaft and having a cruciform cross section, and the recessed portion being formed on the input shaft and having a corresponding configuration to the projecting portion. During the normal operation, the projecting portion and the recessed portion are not engaged with each other and allow the twisting of the torsion bar. On the other hand, when the relative rotation of the output shaft and the input shaft has become larger than a limit angle, the projecting portion is engaged in the recessed portion, thereby functioning to prevent the excessive twisting of the torsion bar.
Further, although the electric power steering apparatus has been used mainly for subcompact passenger cars with relatively light weight, the electric power steering apparatus has also come to be used for compact passenger cars with heavier weight. In such a compact passenger car with heavier weight, the force from the road surface and the steering force tend to become large, so that there is a possibility that the torsion bar becomes twisted to its limit angle, and therefore striking sound occurs frequently due to the collision between the projecting portion of the output shaft and the recessed portion of the input shaft.
In contrast, in the case of an electric hydraulic power steering apparatus which is conventionally used for a compact passenger car and the like, viscosity resistance is produced so as to prevent sudden relative rotation of the input shaft and the output shaft, so that such striking sound has not presented a particular problem.
To suppress such striking sound, it is conceivable to set the limit angle to a large degree such that even if the torsion bar is twisted at a larger angle, the projecting portion of the output shaft and the recessed portion of the input shaft do not collide against each other. However, if the torsion bar is twisted with a large degree, an output signal from a torque sensor increases (or decreases) corresponding therewith, so that there is a possibility that the output signal exceeds a threshold for detecting an abnormality of the torque sensor. If such an output signal, which is outputted from the torque sensor when the torsion bar is twisted with a large degree, exceeds the threshold, an erroneous diagnosis is possibly made that the torque sensor is abnormal although its control circuit is normal, thereby issuing an error signal.